This invention relates to heat exchangers for a Stirling engine and more particularly to parallel flow heat exchangers for isothermalizing the expansion and compression spaces of the Stirling engine.
The ideal Stirling cycle is based on isothermal compression, constant volume heating, isothermal expansion, and constant volume cooling. This theoretical thermodynamic cycle is equal in efficiency to the theoretical Carnot cycle. However, there are numerous aspects of a practical Stirling cycle engine which cause its thermodynamic cycle to deviate from the classical theoretical Stirling cycle, with corresponding reductions in thermal efficiency. For example, the motion of the pistons is usually sinusoidal and therefore the P-V diagram is more oval than the curved parallelogram shape of the classical Stirling cycle P-V diagram. Other deviations from the classic Stirling thermodynamic cycle are introduced by frictional losses in the machine, gas leakage losses around the piston, and windage losses associated with gas flow through the heat exchangers.
One of the most serious deviations of practical engines from the Stirling cycle is a tendancy for the thermodynamic process in the expansion and the compression volumes to be adiabatic rather than isothermal. This results in part because the series arrangement of the heat exchangers causes the gas in the compression volume to be thermally isolated from the cold side heat exchanger, and causes the gas in the expansion volume to be thermally isolated from the hot side heat exchanger. Thus, as the gas expands or is compressed in the expansion or compression chambers, it does so in a gas volume which has already passed through the heat exchanger and is in effect insulated from heat exchange surfaces. Although the walls of the expansion space and compression space are at substantially the expansion and compression temperatures, they do not constitute effective heat exchangers with the gas in the expansion and compression chambers because of the very small surface area to volume ratio. Thus, the gas expanding in the expansion chamber tends to decrease in temperature, and the gas being compressed in the compression chamber tends to increase in temperature. These deviations from the classical Stirling cycle produce degradations in the classical Stirling cycle efficiency.
Another problem with the Stirling engine is associated with the critical length of the series heat exchangers in a reciprocating gas stream. The heat exchange properties between a hot or a cold surface and a gas is a function of the surface to volume ratio and the temperature differential between the heated surface and the gas. To provide a optimum heat transfer, it is necessary to make the gas flow passages very narrow or very long, thereby giving a high surface-to-volume ratio. However, these configurations result in high pressure drops across the heat exchangers, or excessive dead volume. Practical heat exchanger design normally results in a trade-off between the fluid pressure drop across the heat exchanger, the dead volume, and the effective heat exchange, resulting in less than desired performance in all respects.
A piston-displacer Stirling engine normally provides a gas flow path through external heat exchangers and an external regenerator. If the requirements of circulation through an external heat exchanger were not present, however, it would be possible to use a regenerator contained in the displacer which is an ideal use of the displacer volume and minimizes heat loss from the gas circuit. However, a regenerator-in-displacer configuration normally results in low efficiency because the heat exchangers on the two sides of the regenerator are normally in the expansion and compression spaces resulting in poor heat exchange.